The county has a total area of 665 square miles (1,723 km²). Out of this, 654 square miles (1,694 km²) is land and 11 square miles (30 km²) (1.72%) is water.

The highest point is Berlin Mountain, which is 2,818 feet (859 m) above sea level. The mountain is in the town of Berlin. The lowest point is sea level at the Hudson River. The Hoosic River, a tributary of the Hudson River, is in the north part of the county.